1. Technical Field
The present invention relates to polymer electrolyte fuel cells, and in particular, relates to a technique which improves power generation efficiency of polymer electrolyte fuel cells.
2. Background Art
Recently, exhaustion of petroleum resources is a crucial issue, and environmental problems such as air pollution and global warming caused by consumption of fossil fuels have become serious. Under these circumstances, fuel cells have attracted much attention as a clean power source for electric motors in which carbon dioxide is not generated, and such fuel cells are being widely developed and used.
In the case in which such a fuel cell is used in a vehicle, a polymer electrolyte fuel cell in which a polymer electrolyte membrane is used is desirably used since high voltage and large current can be obtained. A membrane electrode assembly (hereinafter simply referred to as an MEA) for the polymer electrolyte fuel cell is produced as follows: a catalyst such as platinum is carried by a catalyst carrier such as carbon black; a pair of electrode catalytic layers is made by unifying the catalyst and an ion conducting polymer binder; a polymer electrolyte membrane having ion conductivity is disposed between the electrode catalytic layers; and a gas-diffusion layer is formed on each of the electrode catalytic layers. Furthermore, a separator which also functions as a gas passage is formed on each of the gas-diffusion layers to obtain a polymer electrolyte fuel cell.
In such a polymer electrolyte fuel cell, a reducing gas, such as hydrogen or methanol, is introduced at one electrode catalytic layer (fuel electrode) through the gas-diffusion layer of the fuel electrode side, and an oxidizing gas such as air or oxygen is introduced at the other electrode catalytic layer (oxygen electrode) through the gas-diffusion layer of the oxygen electrode side. In the fuel electrode, due to the existence of the catalyst in the electrode catalytic layer, protons (H+) and electrons are generated from the reducing gas, and protons migrate to the electrode catalytic layer of the oxygen electrode side through the polymer electrolyte membrane. In the oxygen electrode, due to the existence of the catalyst in the oxygen electrode, protons react with the oxidizing gas introduced at the oxygen electrode and electrons to produce water. Therefore, by electrically connecting the fuel electrode and the oxygen electrode with a lead, a circuit in which electrons generated in the fuel electrode migrate to the oxygen electrode is formed, and an electric current is obtained.
To produce such an MEA, various methods such as a method in which an electrode layer formed on a supporting body is hot pressed to an electrolyte membrane, a method in which an electrode layer is cast on an electrolyte membrane, or a method in which an electrode layer is directly coated on an electrolyte membrane by spraying or the like, have been suggested. Except for the case in which the composition varies in a thickness direction unintentionally, the electrode generally has a single layer structure of a single composition.
To improve the efficiency of such an electrode, various techniques have been suggested conventionally. For example, from the viewpoint that gas concentration is relatively high in an electrode layer of a diffusion layer side and concentration of ionized ion and electron is relatively high in an electrode layer of an electrolyte membrane side, a technique in which composition of the electrode layer in the thickness direction is changed, that is, the amount of supported catalyst at the electrolyte membrane side is increased to increase reaction sites, is disclosed in Japanese Unexamined Patent Application Publication No. Hei 09-180730. Furthermore, a technique in which the amount of electrolyte in the thickness direction is changed to control the diffusion properties of the reaction gas (see Japanese Unexamined Patent Application Publication No. 2001-319663), a technique in which hydrophilicity and hydrophobicity are changed in the thickness direction of the electrode layer to control diffusion of water generated in the reaction (see Japanese Unexamined Patent Application Publication No. 2002-151090), and a technique to control diffusion of both reaction gas and water (see Japanese Unexamined Patent Application Publication No. 2002-164057) are disclosed.
However, the above-mentioned conventional techniques are intended to control reaction or distribution of reactant and product, and there has been no technique in which composition is varied to focus on flow of electrons and protons.